Some inspectors create a special form group for multi-unit buildings in the Custom Form Studio, copying or sharing the various forms they need for a new group there and customizing it to more specifically fit multi-unit inspections. So you could use Custom Form Studio to create a custom form group just for inspecting a multi-unit complex if you want.  You might first Copy your main form group to a new group name for your multi-unit purposes.  Then add additional form sections to that group as desired.  If you need to replicate or repeat existing forms, you could create a new form for the replicated section, and then Insert a subform within it that brings in the original form content again.  You can assign your own custom Form Description in the main form containing your subform.

Or check with vendors on 3D's Marketplace under User Community if you are looking for something already created specifically for multi-units that you can just install and use ready to go.  Some forms on 3D's free Bonus Downloads page may also be useful, perhaps some of the small commercial forms available there, modified somewhat to suit your needs.

Another option is to simply create separate reports, once for each unit, using the forms with which you are already familiar.  Any common areas covered in the first report could be skipped in other reports, since any unused form areas simply wouldn't print.  This might be more convenient if using 3D Mobile apps, since the reports would each be smaller to work with.
If you create separate reports for different units, you may link each to the same job in Office Management by creating additional reports using available Job Name- Invoice item combinations so the same job information can merge into each of them, and they stay organized with your job.

For instance, if a general "Inspection" invoice item could be used for starting a 2nd report besides the main job name likely used for the first report. Or you might add an Extra Unit 2 Invoice item, and so forth to handle the additional units. (an additional fee could be included or without pricing- items without pricing may even be set non-visible so they don't print on your client's invoice or receipt)

Then you may print or email each unit report separately, or if you wish to deliver all the separate reports together, you may use the Attachments feature to do in this manner:  Open one of the reports.  Print it to a PDF file on your computer. Open the other report.  In the lower Attachments pane, click the Attached Files tab.  Right-click in the Attached files pane and Add New.  Change the Files of type menu to list PDF files.  Navigate to the folder where you saved your PDF for the other report, and select it.  Now when you email the report you have open, it will automatically include the other report PDF in the email that is sent out.